土壤渗透水微量元素对地表环境的响应及在洞穴现代沉积物研究中的应用

Trace elements in speleothems record the information of paleoclimate and paleoenvironment. Although multi-proxies have been referred to, the utilizing of trace elements was not as much as the oxygen and carbon stable isotopes because of the complexity of the resource and numerous influences for the trace elements. It is the premise work to understand the sources of elements in cave drip water and speleothems and to comprehand the mechanism of geochemical signals trancefering in the geological processes when using stalagmite trace elements to reconstruct past environment. We will establish monitoring sites above Furong Cave at Wulong County in Chongqing to monitor the air temperature, precipitation and the temperature, humidity and the concentration of CO2 of soil. The concentration of Ca, Mg, Ba, Sr, Fe, Mn and other geochemical indicators in the soil infiltrating water will be detected. Tracer experiment will be used to understand the transfer time that groundwater flows in the soil and rocks. The main content of this project is to study on the effection of atmospheric environment, soil chemical composition and vegetation to the geochemical composition of soil infiltrating water. The monitoring data will be studied to combine with the geochemical character of dripwater and modern speleothem. The cave monitoring system of Furong Cave from rainfall- soil- soil infiltrating water - rocks- dripwater- modern speleothem will be established. This project will improve our understanding about the response of the geochemical character of soil infiltrating water to the environment change. The transport mechanism of groundwater in the soil and bedrock will also be studied. This work is helpful in understanding the main source of trace elements in speleothems and to know what factors affect the distribution of the elements. It will undoubtedly benefit to the scientific explanation of trace elements in speleothems and valuable to robust reconstruction of paleoclimate and paleoenvironment.

了解洞穴滴水和洞穴沉积物元素的来源及各种地质过程中地球化学信号的传递机理是利用石笋微量元素重建过去环境变化的前提。本研究计划在重庆市芙蓉洞上部建立观测点，对大气温度、降水量、土壤的温度、湿度和CO2浓度等进行监测，并分析芙蓉洞上覆土壤渗透水Ca、Mg、Ba、Sr、Fe、Mn等元素浓度及其他地球化学指标的变化，以研究大气环境、土壤化学组成和植被对土壤渗透水地球化学组成的影响为主要内容，运用示踪实验了解地下水在洞穴上覆土壤和基岩中的运移时间，同时结合本团队在芙蓉洞对洞穴滴水、现代洞穴沉积物微量元素的分析数据，完善大气降水-土壤-土壤渗透水-基岩-洞穴滴水-现代洞穴沉积物的洞穴立体监测系统。项目的开展有助于了解土壤渗透水地球化学性质对地表环境变化的响应和地下水在土壤与基岩中的运移过程，从而掌握洞穴沉积物微量元素的主要来源与控制因素，合理解释芙蓉洞石笋微量元素记录的环境信息。

通过三年来对芙蓉洞洞穴上覆土壤渗透水、土壤CO2、洞穴滴水地球化学性质、洞穴环境每月一次的连续监测，本课题组圆满完成了项目拟定的各项研究内容，主要的研究发现如下：（1）土壤渗透水量与大气降水量之间具有较好的一致性，说明土壤渗透水能快速响应大气降水量的变化。（2）芙蓉洞上覆土壤pCO2与气温和降水量的季节变化相对应，夏秋季节浓度高，冬春季节浓度低；气温对土壤pCO2的影响更明显；洞穴与土壤pCO2变化趋势相似，但受土壤和基岩厚度的影响，存在2-3个月的滞后期，且洞穴pCO2能反映降水量的双峰特征。（3）滴水δ13C值均呈现夏秋季节偏轻、冬春季节偏重的季节变化规律，与土壤pCO2变化规律相似，说明土壤CO2是滴水中碳的主要来源；滴水δ13C值在冬春季节为-2.0‰～0‰，说明在降水量较少的冬春季节，基岩是影响滴水δ13C的重要因素。（4）2009-2013年期间，每年进入10月后，滴水δ13C值迅速偏重，到翌年4月迅速偏轻，与同时期中国西南地区的干旱气候事件相对应，说明芙蓉洞滴水δ13C值可以及时反映较大区域范围的极端气候事件。（5）土壤渗透水中Mg/Ca、Mg/Sr比值在4 月和7月分别出现峰值，主要受到土壤渗透水的滞留时间、气温和降水量以及生物作用的影响。4月的峰值是由于前期干旱导致水-土作用时间延长，使渗透水中的元素浓度普遍升高所致；7 月的气温升高使土壤中的盐类物质溶解速率发生变化，以及土壤层生物活动加强、土壤CO2 含量升高，增强了土壤水溶蚀能力，使Mg/Ca 和Mg/Sr 比值又出现峰值。 .通过综合分析多年监测数据，认为芙蓉洞现代沉积物的碳主要来自于土壤CO2，洞穴滴水δ13C值可以及时响应地表环境变化，洞穴沉积物的δ13C可作为记录地表降水以及土壤生物过程变化的替代指标。